I have seen kits at our local pet supply store, and have heard of vets that will take swabs for genetic testing to determine the makeup of a family dog.

These tests are usually marketed to people with mutts to determine how many different "flavors" our dogs are made of. Results are supposed to be fairly conclusive and it might be a way to go if this question is really making you crazy, but I think the tests run something like $60-100.

We considered it for our pup Gus as a fun thing to do but decided a $60 kit was too much to spend for a fun test since it doesn't really make any difference to him. (I'm a biologist...so perhaps my definition of "fun" is a little skewed!)